The present invention relates to a data communication system, and in particular, relates to such a system which provides an inexpensive data communication system for a small area, like a data communication system within a building, a campus, or a plant ground.
A data communication network for such a small area is called a local network, which has usually a host computer, and a plurality of terminal apparatuses each coupled with said host computer. Recently, a new concept of the local network has been proposed, and in that new concept, no host computer, nor an exchange system is used.
One of prior local networks is the ETHERNET developed by Xerox Corporation, and that system is disclosed in the Japanese laid open patent publication 114804/51, which claimed the convention priority based upon the U.S. application Ser. No. 563741 filed Mar. 31, 1975 now U.S. Pat. No. 4,063,220.
The ETHERNET system is shown in FIG. 1, in which the reference numerals 11 through 1i are data terminals, 61 through 6i are communication control apparatuses, 3 is a coaxial communication cable, 41 through 4i are cable driver/receiver circuits, and 5 is a termination circuit for the impedance matching of the coaxial cable 3. The communication in FIG. 1 is accomplished using the concept of CSMA/CD (Carrier Sense Multiple Access with Collision Detection).
In FIG. 1, when the data terminal 11 forwards data, the related communication control apparatus 61 composes a packet with that data, and then, the driver/receiver circuit 41 monitors if the cable 3 is available or busy by detecting a carrier signal on the cable 3. It should be noted that if the cable 3 is busy, a carrier signal transmitted by another terminal is detected. If the cable 3 is not busy, the data packet prepared by the communication control apparatus 61 is forwarded to the cable 3 through the driver/receiver circuit 41. If the cable 3 is busy, the data packet is forwarded to the cable 3 after the cable 3 becomes available. The driver/receiver circuit 41 monitors also the status of the cable 3 while the apparatus 61 is transmitting the data packet, and if another terminal transmits a data packet simultaneously (collision of data packets), the driver/receiver circuit 41 stops the transmission of the data packet, and re-transmits that data packet after a predetermined pause. The communication control apparatuses 62 through 6i monitor the data packet on the cable 3 through the related driver/receiver circuits 42 through 4i, and when the communication control apparatus recognizes a packet addressed to its own terminal, then, that communication control apparatus takes that packet, which is transferred to the related data terminal, otherwise, the data packet is disregarded.
Therefore, the ETHERNET has no host computer for the control of the whole network, but each terminal may control the communication with its own decision, thus, the structure of the network is simplified as compared with a prior network which must have a host computer. Further, by the use of a coaxial cable which can accomplish a wideband transmission, a large capacity of communication in which several thousand terminals may communicate simultaneously is obtained.
However, the ETHERNET has disadvantages as follows. First, a coaxial cable must be newly installed, and therefore, when the number of terminals is small, the initial cost for each terminal is relatively high. Secondly, a driver/receiver circuit must be directly attached to the coaxial cable so that errors of the impedance matching of the cable are prevented, and therefore, each data terminal must be located near the coaxial cable. Further, when a coaxial cable is in trouble or broken, the whole communication is completely stopped.